Orthodontic and other dental laboratories provide services for the custom-fabrication of dental appliances for an individual patient according to a treatment plan and a prescription provided by an attending orthodontist or dentist. A poured and cured stone replica of a patient's mouth structure, such as an initial malocclusion for example, is typically provided by the doctor for the laboratory's use.
The dental technicians at the lab first modify the stone model by cutting the mal-positioned teeth free of the model and the adjacent teeth. Next, a technician repositions the teeth on the model semi-rigidly into desired, ideal positions as specified by the doctor and as determined by the doctor's diagnosis and subsequent treatment plan. After the stone model has been modified or “corrected” in this manner, the model will be positioned within an appliance-forming machine where a sheet of thermo-formable elastomeric material is typically “sucked-down” or pressed over the stone model(s).
The use of thin sheets of various thermo-formable plastics including vinyl and olefin-type materials has been adopted for many current appliances. This, along with an efficient forming process involving the rapid use of pressure, vacuum and heat, has replaced prior molding and casting processes. The use of these materials, including polypropylene in sheet form and in thicknesses of about 1 mm (before thermoforming), eliminates the time-consuming steps of mixing, catalyzing and curing and optionally heat-curing in a pressure flask, as was required by natural rubber, medical-grade urethane, silicone and vinyl silicone series materials.
Polypropylene (PP), for example, is often used in making dental appliances because it is non-reactive chemically and thoroughly biocompatible. However, PP alone may exhibit plastic creep over time, resulting in minor, but undesirable changes in the appliance's shape and it may also lack the level of clarity desired by some patients. Likewise, PP's characteristics for making a dental appliance by thermoforming is adequate, but not ideal for consistent, accurate products. This may prevent cost effective production of consistent, high-quality appliances demanded by dentists and their patients.
Better thermoforming characteristics are found in medical grade copolyester sheets, which are also used to make dental appliances. However, dental appliances made from copolyester sheets alone have a tendency to become brittle over time. As a result, a patient wearing such an appliance who has a tendency to grind his teeth may grind into or through the appliance, reducing or eliminating the appliance's effectiveness.